The present invention relates to grinding apparatus of the disc type for grinding pulp material, preferably lignocellulose-containing material in the form of chips. The grinding members rotate relative to one another and are provided with grinding surfaces located at a distance from the center of rotation. Said grinding surfaces define therebetween a grinding space for the material to be ground. The grinding space preferably extends at an angle to the axis of rotation. An especially important field of application of the invention are defibrating machines or refiners for manufacture of fibre or paper pulp from wood chips or other similar cellulose-containing material.
Grinding apparatus of the disc type have hitherto been formed with a grinding space between the grinding members with main extension in radial direction, said space extending from a central feed-out zone for the starting material where the centrifugal force is relatively low. The centrifugal force acting on the material or pulp stock is then increased very strongly with increasing radial spacing from the centre of rotation. In order to prolongate the period of stay of the material in the outer portion of the grinding space zone it has been proposed in the co-pending U.S. patent application Ser. No. 877,809, filed Feb. 17, 1978, now U.S. Pat. No. 4,253,613, to design the grinding space in the outer grinding zone with extension in axial direction so as to permit a fraction only of the total centrifugal force to actuate the pulp stock in the direction of flow determined by the outer space portion. Whereas measures have been taken in the radially outer portion of the grinding space to restrict the speed of flow of the pulp stock towards the peripherally outer outlet side of the grinding zone. These measures are still hampered by the phenomenon that the actuation by the centrifugal force in the central feed-out zone is low so that the supply to the outer grinding zone is not so intensive as desirable for operation of the grinding apparatus with its highest capacity. Therefore, application of mechanical devices for promoting said feeding advance have been tried in the central zone of the apparatus, but these devices have proved to be not so effective as presumed i.a. because they require to be replaced relatively often since they are subjected to rapid wear.